Sulfuric acid purification



Sept. 8, '1959 s. R. sTlLEs SULFURIC ACID PURIFICATION Filed July 26. 1955 m9 ,s mo. vo. m E :l l on T 0C'. O v- T. S m R. V L m mi M Dn n 5 mw f mw W \.w-

A 5 E. vmxm f ww om ww. mw.. wm. mm wm v N A Ow I 9v 02:23@ Nw wwwmmmmm mzamox wm 2 l. f m MMI i w .v\ om, o@ wm mw vv E mm l 275.5 mm mm mwa Nv YJ ATTORNEYS United States hatent SULFURIC ACID PURIFICATION Samuel R. Stiles, Cresskill, NJ., assignor yto .The M. W. Kellogg Company, Jersey City, NJ., a corporation of Delaware Application July 26, 1955, VSerial No. 524,537

4 Claims. (Cl.` 23-172) This invention relates to an improved method of purifying contaminated sulfuric acid, and more particularly, it pertains to a method of purifying sulfuric acid by crystallization whereby serious operating diiiculties are overcome.

Spent sulfuric acid contaminated with polar` and/or non-polar impurities can be purified by crystallization of the spent acid and subsequent separation of the acid crystals from the mother liquor containing a substantial proportion of the impurities. Under the conditions of crystallizatiomit is found that the mother liquor is highly viscous and, therefore, the acid crystals in the mother liquor tend to agglomerate and cause serious operating dilculties inthe system. This problem exists to a lesser extent in those systems utilizing an organic liquid spacer as the medium for crystallization, however, in view of the immiscibleA nature of the organic liquid spacer, there is still the tendency for the acid crystals to agglomerate if the mother liquor is allowed to separate substantially from the organic` liquid spacer. By -means of this invention, a methodof operation is proposedwhereby these difculties are overcome substantially.v

An object of this invention is to provide an improved method for the purification of sulfuric acid contaminated with polar and/ or non-polar impurities.

Another object of this invention is to provide an improved method of purifying. sulfuric acid contaminated with polar and/ or non-polar impurities by crystallization whereby operating diculties associated withY the agglomeration of acid crystals are substantially overcome.

Other objects and advantages of this invention will become apparent from` the following description and explanation thereof.

The present invention is concerned with a'process in which sulfuric acid is crystallized in a crystallization zone to produce'mother liquor and crystals of sulfuric acid, and the processis operated to prevent agglomeration of acid crystals by the method whichv comprises crystallizing the sulfuric acid under conditions of turbulent agitation in the crystallization zone sucient to prevent agglomeration of .the crystals.

1n those cases Where an. organic or inorganic liquid spacer is employed inthe purification treatment such that sulfuric acid crystals, mother liquor and liquid spacer are present in the crystallization zone, operating dii-l culties are substantially. overcome by the method which comprises crystallizing the sulfuric acid under conditions of turbulent agitation in the crystallization zone sucient to prevent substantial separation of mother liquor from the spacer and thereby preventing agglomeration of the crystals.

In another aspect of this invention, where spent sul; furic acid is crystallized to produce mother liquor and acid crystals in a crystallization zone and where a lslurry of acid crystals and .mother liquor is passed from the crystallization zone to` a separation zone by means of a transfer zone, the improvement comprises crystallizingv ice the spent acid under conditions of turbulent agitation in the crystallization zone thereby producing a slurry of acid crystals and mother liquor without significant agglomeration of crystals, and passing the slurry from the crystallization zone to the transfer zone forfsubsequent passage to the separation zone at a linear velocity in excess of about 15 feet per second.;

The process of this invention is applicable to any spent sulfuric acid containing polar and/ or non-polar impurities. It should be understood that for the purposes of this invention Water isl considered as a polar impurity by reason of its attraction or aiinity for sulfuric acid. The spent acid may contain a total acidity ranging considerably in value, however, frequently, this; invention will iind use for the purilication of a spent acid containing at least about total acidity, determined as H2804, and more particularly, the acid will contain a total acidity of at least about and as high as 96% or higher on the same basis. Thev spentacid can be derived from any source, namely, an ,acidi resulting from the treatment of lubricating oils, gasoline and naphtha, for the removal of mono-olenic or diolenic materials, with or without the removal of sulfur compounds, or the acid can be derived from a processi/in which a mono-olefin is alkylated with an isoparaflin in the presence of sulfuric acid.

In the latter application, the spent acid contains alkylate or the desired .product and acid esters or mono-alkyl sulfate, the di-ester or dialkyl sulfate, with or without water. The water concentration of the spent acid can be reduced to zero value,or to a maximum of 7% of water, by treatment with SO3'or filming sulfuric acid. Generally, the spent acid contains about 0 to about 5% or 0.01 to 2% of non-polar impurities and about 0 -to about 10 or 0.1 to 5% of polar impurities. In the case of spent alkylation acid, thej acid ester is present in an amount of about l to about 6%, the di-esteris'about 0 to about 8%, and water is about 0 to about 4% of the total spent acid, on a Weight basis. The non-polar impurity is, for example,'the alkylate polymer or di-ester, e.g., dialkyl sulfate, etc.; whereas the polar impurity is the acid ester or mono-alkyl sulfate, water, etc.

The crystallization ofthe spent acid takes place at a temperature ranging from about F. to about 50 F. ln the case whereA crystallizationV takes place in the absence of a liquid spacer, it `is preferred toemploy a temperature of about 0 .to about; 50 F. by reason that the resultant mother liquor is highly viscous, and it would Abe desirable to safeguardfagainst the strong tendency for the acid crystals to agglomerate within the mother liquor, which undergoes a high increase in viscosity at lowtemperatures. i When the` present invention is practiced by using a liquid spacer, it ispreferred that the crystallization temperature range from about 50 F. to a temperature below 0 F. or,v generally, the temperature of crystallization can be less than 0 F. The use of the liquid spacer tends to oiset the elect ofthe highly viscous nature of the mother liquor, thus making possible the useV of lower, temperatures at which there is an increase in yield `of acid` crystals. The pressure of the crystallization zone can vary over'a wide range, namely, it can be subatmospheric, atmospheric or superatmospheric, for example, theV pressure of the crystallization step can be about 2 mm. Hgabsolute to about 100 p.s.i.g., although,.more usually, it willvary from about 2 inches H2O to about 1 atmosphere.

The liquid spacer` to..be used for this invention is any.

organic or inorganic liquidv which is non-reactive and immiscible with sulfuric acid and it is preferably, although' hydrocarbon, preferably Ya light aliphatic hydrocarbonv containing 1` tol-5 carbonatoms, halogenated aliphatic hydrocarbons, halocarbons, aliphatic ethers or an inorganic liquid, e.g., sulfur oxide, viz., SO2, etc. The quantity ofliquid spacer employedA ini the crystallization zone relativel to the quantity of sulfuric acid beingi processed on aY volumetric'basi's: is: about 011`= to abouti l`00ip'arts per part of acid, although more usually about 055'- to about 5'0 parts of', spacerperpart of acid are employed'` It is also contemplated within the scope ofthepresent inventmni to employ a volatilizable organic orinorganici liquid asan auto-refrigerant'. In connecti'on-y with' auto-refrigeration, the volatilizable liquid is fed' tol the crystallizati'onI zone au a: rate and under suchl conditions withinA the zone that a. portionI of the spacer'liquid islylolatilized?` or vaporized thereby cooling thef crystallization mediuml or' acid tothedesired temperature level. For-thispurp'ose, lighter parafns containing.- abouty 11 to 4 carbonatoms arel especially suited such. as,- for.I example, methane, eth'a'ne,A propane, n-butane, isobutane, etc.

Optionally, they crystals` of sulfuricacid,. following separation'y from the-mother liquorgwithmr without spacer material, are subjected to a-washi'ng,Y operation. In the-V washing. operation, any of4 the= liquid spacersf describedI above-can be employed. Howevenitf should be bornein mind that the use of ai washA liquor-"which is dissimilar with the spacer material' employed' in the crystallization step may. serve to complicate thel process, and? for that reason,.it is preferredr than the spacer: material be of the same' compositionias the wash liquor. The washing step is-conducted at a; temperature of about' -50 F. to about2 o 50?' F.,. however, thev important; factor being thatthe temperature is'below. the leveliat' which the crystals melt. The washing step is conducted byusing about l toabout lOOfl'parts by'volume ofzwashf liquor, more usually about 5 toabout 25 parts by volume oflwash liquor per part byy volume of acid crystals;

In accordanceiwith the improvement offthis invention, the;crystallizationzof spent sulfuric acid,with` or without;

the use of any organic or inorganic liquidl spacer, i's con'- ducted in afcrystallizationzone *under conditions of' turbulent agitation; By. turbulent agitation, it is meant, forexample, in the caseof crystallizing sulfuricacidv in the absenceof al liquidi` spacer, that a" slurryoff acidcrystals and mother liquor is formed withoutsubsequentl agglomeration of acid' crystals which would causeoperatingdiflicultiesby, plugging theexitline fromthe crystallizationzone. Generally,the'agitationshould be conducted under conditions sufficiently turbulent to-preventi the formation of agglomerates or crystal-masses having anapproximate length or average diameteroff more thany 1/a` of aninch.y In. a. preferred aspect-v of? this invention, by suitable agitation ofA the crystallization medium; there is little-'agglomeration-and the sizeof the crystals or agglomerateswill appear to be in-the range ofaboutI 1/100 toabOut'J/l of an inch. in diameter. When theliquid-V spacer is employed, an alternative criterionto followiis that the-agitation: should besufi'cient' to-prevent substantial separation: of the motherV liquorfrom-the liquidi spacer; The tendency. for the acid crystals to vagglomerateinthe'- liquid spacer is substantially lessf than in the mother`v liquor, consequently,vv

substantially.l and little difficulty* may beexperienced' in' the operation ofthe unit Itis. preferredi todisperse the mother liquor andIcrystals-in the-liquid spacer.' Similarly, inthe'. case where-crystallization is conductedin the absence of the organic liquid spacerthe* size of the agglomcrates: or crystals shouldJ be' in. tlie'range specified hereinabove; otherwise operating difficultiesV will be experienced, due `to plugging of 'the outlet' lines, `etc;

In` the-operation of the present invention, itis alsocontemplated recycling` a' portion of the slurry of, acid crystals in--motlier liquor, withorwithout organic liquid by maintaining adisp'ersionor` emulsion ofV mother` liquor. in the liquid? spacer, 0rvice versa; theA tendency for the' acid Vcrystals to agglomerate isreduced' 4. spacer, to the crystallization zone for the dual purpose 0f (l) lseeding or initiating the crystallization of the incoming sulfuric acid, and (2) enhancing the turbulent conditions maintained within the crystallization zone. For this purpose, the recycle of slurry on a volumetric basis relative to the quantity of acid being fed for crystallization thereto is about 10` to about 1000: l, more usually about 50 toy about 250:1-, It is preferred that the slurry of acid.- crystals be recycled to the crystallizationv zone atl arate sucient to circulate at least six times the volumetric holdup of the crystallizer per minute andl up to twenty times the volumetric holdup per minute. The condition of turbulent agitation can be produced' within the crystallization; zone by mechanical agitation of a conventional Istirring mechanism, such as a driving shaft containing one or more propellers or paddles positioned along the vertical length thereof and providing sufcient turbulent action to effect the result' desiredl Another method by which the agitation cany beaccomplished is to inject; one or more streams of liquid2 spacer and/or spent acid andVor slurry of crystals into the crystallization zone at linear velocities in excess of about 50y feet per second, and preferably at linear velocities of about toy about 400 feet= per secondi In` onemethod' of oper-ation, the spent acdis introduced'- intox the top partof the crystallization zone, consequently, there is a strengv tendencyv for agglomeration ofv crystals to occur within the bottom partof the crystallization zone.y To safeguard against undue agglomeration ot acid crystals, a jet or a plurality of jets of liquid spacer is-fedl at the velocities specified: upwardly throughor directly into the descendingmass of crystallization medium thereby-f creating the turbulent agitation which isfdesirable-f'or this'step of the process. In the: idealI condition, thecrystallizaticn medium will'appear asa homogeneousrsluiry'of acid-crystals dispersed throughout ther crystallization medium which maybe the mother liquor with; or Withouti liquid spacer. At thewithd'rawalfpoint of crystabslurry from-the crystallization zone,.it.is desirable to inject. organic liquid' spacer in countercurrent flow to: the: outgoing crystal slurry or any other ldirection relative thereto which will' effect the desired turbulent condition.. Alternatively, a portion of slurry maybe withdrawnfromithe-crystallizer, aty the top or side or any other pointy spaced.I from the outlet and injected intothe crystallizer as; just. described fon thel liquidF spacer to prevent` agglomeration; at inthe crystallization zone.`

Any combination of' thef various: previouslyV described methods for obtainingithe. required. agitationv canbe practiced without departing from the scope of this invention. For example, mechanical agitationi can be used' simultaneously with high velocity injection of.' a lluid stream into the crystallization'zone; The-:source of. the fluid injected is preferably a slurry.y withdrawn from.y the lower portion of the crystallizationfzone;

Another important problem*V with.l respect` tol the operationl of asystems involving: the: purification'l of sulfuric acid by crystallization is thatz there is a tendency for the sulfuric acid crystals t'o agglomerato within the transfer zones or lines, interconnecting,l fon example, the crystalthe outletl and lization zone with' the. separations zone: wherein the' acidcrystals are separatedV from the` mother. liquor, with or without liquid-v spacer, andf the transfer` zone. or line by which the slurry of acidcrystals is:recycled.to thezcrystalL lization zone for the. purpose describedhereinabove. it.

GL ing which forms a part of this specification and illustrates a specific embodiment of the present invention.

In the drawing, olefin feed containing 20% propylene and 80% butylene is fed from source 5 at the rate of 3240 b.p.d., and this stream is divided such that one-half of the same is fed through line 6 and the remaining portion is fed through line 7. These split streams of olen feed are fed through mixers 17 and 18 of alkylation reactor 9. Alkylation reactor 9 contains baffles 10, 11 and 12 which divide part of the reactor into two reaction zones, namely, 14 and 15. The reactant material in zone 14 is agitated by means of mixer 17; whereas in section 15 the reactant material is agitated by means of mixer 18. Isoparafn feed, e.g., isobutane, is fed to section 14 at the rate of 11,600 b.p.d. by means of line 20. Sulfuric acid containing 96% free acid is fed to section 14 at the rate of 1000 b.p.d. by means of line 21. The conditions within sections 14 and 15 are sucient to effect alkylation of butylene with isobutane and these conditions include a temperature of about 35 F. and a pressure of about 5 p.s.i.g. It should be understood that the conditions and reactant materials employed in this example are merely illustrative, because the present invention is adapted to the alkylation process broadly involving an olefin contain* ing, for example, 3 to 6 carbon atoms and an isoparaflin containing about 4 to 8 carbon atoms. The relative portion of isoparaffin to olefin varies from about 50 to about 5000 mols of the former per mol of the latter. Based on the olefin feed, the sulfuric acid charged to the reaction zone is about 0.3 to about 15 parts by volume per part of olefin charged per hour thereto. The sulfuric acid can have a total acidity ranging from about 90 to about 100%. The reaction can be conducted at a temperature of about to about 100 F. and a pressure of about 1 atmosphere to about p.s.i.g.

As a result of the conditions in sections 14 and 15 of the alkylation reactor, the total reaction mixture passes over partitioning baffle 12 and enters a settling zone 23. In this settling zone, a final separation of acid from the reaction product is elfected. Consequently, the reaction product being lighter, generally exists as the supernatant phase; whereas the acid is the lower phase. The acid is withdrawn from the bottom of section 23 by means of line 24, and it is pumped by means of pump 25 through line 26. A portion of the acid is recycled to the alkylation reactor 9 by means of lines 27 and 21; whereas the remaining portion, namely, about 200 b.p.d. passes through line 28 and it is earmarked for purification in accordance with the technique of the present invention to be discussed hereinafter. The supernatant layer of reaction product passed over baffle 30 from section 23 and enters a surge zone or section 31. The reaction product containing alkylate is discharged from the'bottom of section 31 by means of line 33, and it is further processed for the recovery of alkylate in a recovery system (not shown). By virtue of the conditions prevailing in the alkylation reactor 9, part of the hydrocarbon reactant is Vaporized in order to maintain the desired temperature therein. The vaporized hydrocarbon is withdrawn overhead from alkylation reactor 9 by means of line 34, and it passes to a separator 35. Any liquid contained in separator 35 is discharged from the bottom thereof through line 36, and it is returned to the alkylation reactor. The vaporized hydrocarbon in separating drum 35 passes from the top thereof through line 38 and then it is compressed to a pressure of about 90 p.s.i.g. by means of compressor 40. Following the compression stage, the compressed hydrocarbon passes from compressor 40 to a cooler or condenser 41 by means of ilne 42. The condensed hydrocarbon passes from condenser 41 through line 43 which leads to the top of a surge drum 44.

The condensed hydrocarbon is withdrawn from surge drum 44 by means of line 46 and a portion thereof, namely, 4000 b.p.d., is passed through line 47, which leads to pump 48, and thence it is pumped to a depropanizer column 50 by means of line 51. 1n the depropanizer column the top temperature is maintained at F.; Vwhereas the bottom temperature is maintained at F. The pressure in the top of the tower is 185 p.s,i.g. Propane vapor is discharged from the top of the depropanizer column 50 by means of line 53. A portion of the propane vapor, at the rate of 1225 pounds per hour, is passed from line 53 to line 54 and the utilization of this stream will be discussed hereinafter. 38,000 pounds per hour of propane vapor is passed through line 53 to another line 55, and this material ows to a condenser S6, and thence to a surge drum 57 by means of line 58. The condensed propane in surge drum 57 is discharged from the bottom thereof by means of line 60, and this material is pumped by means of pump 61 such that about 55 b.p.d. are passed through line 62 and utilized in a manner discussed hereinafter; whereas 4400 b.p.d. are recycled to the depropanizer column 50 by means of line 63. The remaining portion of propane vapor is discharged from line 53 at the rate of 4500 pounds per hour through line 64. The net production of propane is yielded from the system by means of line 65. C4 and heavier hydrocarbons which are present in the feed to the depropanizer column is discharged from the bottom thereof by means of line 66 at the rate of 2950 b.p.d.

For the purification of the spent sulfuric acid from the alkylation system and flowing through line 28, it is desirable to add furning sulfuric acid thereto in order to reduce the Water concentration. Originally, the spent acid contains 2% of water by weight and furning sulfuric acid is combined with the spent acid by means of line 70 at the rate of 20 b.p.d. in order to reduce the water concentration below about 0.5% by weight. It should be understood that it is not necessary to reduce the water concentration to zero in order to obtain a reasonable recovery of purified acid. The combined stream of furning sulfuric acid and spent acid passes through line 72, and then this material is' combined with recycled sulfuric acid, to be discussed in greater detail hereinbelow, at the rate of 20 b.p.d., by means of line 73. The combined sulfuric acid streams pass through line 74, and thence into the crystallizer 77. The acid stream is sprayed into an organic liquid spacer, which also serves as the auto-refrigerant comprising propane and isobutane. The quantity of spacer added to the crystallizer is sufiicient to make up what is discharged therefrom, and the concentration of propane in the spacer varies with the temperature required for crystallization.

The crystallizer 77 contains a stirring mechanism 78 illustrated as a vertical shaft containing propellers 79. The purpose of the stirring mechanism is to maintain the crystallization medium in a turbulent state of agitation to prevent stratification or separation of the spacer from the mother liquor. The temperature in the crystallizer is maintained at 35 F. and lat -a pressure of 5 inches of water. As a result of these conditions, a portion of the .spacer is vaporized, consequently, cooling the crystallization medium. The vaporized spacer is discharged from the top of the crystallizer 77 by means of line 80. It should be noted that one of the propellers of the stirring mechanism 78 is positioned within the bottom part of conical section 82 of the crystallizer in order to prevent bridging or plugging of the effluent material due to agglomeration of the sulfuric acid crystals. The sulfuric acid crystals present in thespacer and mother liquor tend to agglomerate due to the highly viscous nature of the mother liquor. A slurry of spacer, crystals and mother liquor is Withdrawn from the bottom of conical section 82 by means of line 83, and it is transported by means of pump 84.

The pumpedslurry is passed through line 85 at the rate of 8500 b.p.d., and this stream is divided such that a portion, namely, six times the volumetric holdup per minute is recycled to the crystallizer by means of line 86. In this regard, the volume of slurry recycled to the crystallizer relative `to ,the ,volume of Spent acid which is -processed to vthe crystallizer by means Vof line 74, stands in the ratio of .about 50 Vto about 500 parts of slurry per part of acid feed. The recycling of slurry serves two purposes, namely, it .assists in creating a turbulent state in the crystallizer, .and ,also it lServes to seed or initiate .crystallization of the incoming sulfurie acid. The remaining portion of slurry which gis passing through line 85 is passed through line 87. 1t ,is important to note that ytheslurry of acid crystals, mother liquor and spacer should ilow :through Llines 85, 86 and S7 at a linear velocity in `excess of .feet'per-second,.otherwise there is a tendency for .the acid .crystals toagglomerate and thus plug these lines. In this example, the linear velocity of material in lines 85:, .86 and 87 is 30 feet per second.

The net production of acid crystals Vpassing through line 87 is passed to a centrifugal .filter ,shown schematically as` 90. This ffilter is of well-.known `design and it comprises essentially a rotating .drum (not shown) upon which the acid crystals are `deposited as a cake, and the filtrate passes .to the .interior thereof for subsequent discharge from vthe filter. .During the course of its operation, wash liquor is contacted with the cake of cid crystals, and thereby serves to remove any sorbed or occluded impurities from the crystal cake. With respect to the drawing, the wash liquor ,is of the same composition as .the auto-.refrigerant and it is fe'd to the centrifugal tilter by meansof line 91 at the rate of 0.1 to 1.0 gallon .per second. The wash liquor ,has a temperature of 35 F. The filtrate vfrom .the centrifugal iilter 90 is .discharged therefrom by means of line 93, and this material .is tpassed to separator 94 wherein the hydrocarbon material, namely, propane-,isobutane is discharged overhead by means of line 95 and the mother liquor is discharged from the bottom thereof by means of line 96. In this operation, b.p.d. of mother liquor are produced. The motor vliquor at this stage ,of the operation is at a temperature of F. The .motor liquor `is passed from line 96 to a holding or settling vessel or ,zone 97 equipped with a heating coil 98. The mother .liquor has a .residence time of 3,0 minutes within the settler 9.7, and the temperature within the settler is maintained at 50 F. It should be understood, however, that the settling time or residence time of the mother liquor in settler-97 can range from about 1 to about 5.000 minutes and the temperature can vary from about 0 to about 150 F. As a .result of the conditions ,prevailing in settler 97, a supernatant phase or layer 4of hydrocarbon material is formed, and this material is the non-polar impurity which, `by analysis, vis found to be an alkylate product. The alkylate product is discharged from the .settler 97 by means of line 100 at the -rate of 20 b.p.,d. Initially, the mother liquor being fed to the settler 97 has an acidity of 7.0%. By virtue of the separation in settler 9,7, the resultant acid phase has a titratable acidity of 94% :or a free acidity of about 90%. The acid of increased concentration is discharged from the bottom of settler 97 by means of line 102. A portion of the acid in line 102 is discharged from the system by means of line 103 .and the remaining portion .of acid passing through .line ,102 flows into line 104 and thence, by means of pump 105, .it is recycled to the `crystallizer by means of line 73, previously mentioned. It is important to note at this Apoint that the temperature .condition within settler 97 should be suiiiciently ,high to avoid the formation of .crystals therein, otherwise difficulties may occur in the handling ofthe acid for recycle tothe crystallizer. Further, the temperature is Ipreferably below about 150 1?.,.because there may -be-atendency for the acid to decompose .at higher temperatures, and therefore, increase the lossof acid through evolutionof sulfur oxide and water.

The spacer ,material which 4is .dischargedfrom separator 94 is passed to a collection zone or decanter or holding vessel 108. 4Collector 108 serves as a means for collectingallof thespacer streams, both vapor and liquid, which are .discharged from .the lvarious ipieces of apparatus in the process, and .thereby a fairly uniform composition of -organic liquid spacer is employed in various stages of `the operation.

Theacid crystals which are separated from the slurry by means ofcentrifugal filter are discharged by ymeans of conduit 1.10 vinto Va Ymelting zone or melter 111. The temperature in the melter is maintained at 50 F. by removing or discharging melted or .liquid sulfuric acid from .the bottom thereof .by means of line 113 and transporting .thesame by meansfof pump y114 and line 115 to a heat exchanger 116. .inthe heat exchanger, the liquid surfuric acid of about .total acidity .is heated from a temperature of 50 .F. to about 70 F. The heated sulfuric acid is discharged from the heat exchanger 116 by means of line 11.7 ,and a portion thereof, namely, about 1200 b.p.d. is recycled to the melter 111 by means of line 120. The remaining portion of -sulfuric acid is discharged from line 1.17 to line ,121, and this material is transported to line 21, whereby it is subsequently utilized in the akylation reactor. Anyorganic liquid spacer which is Vaporized within melter 111 or centrifugal ilter 90 is discharged therefrom `by means of lines 123 and 124 respectively.

The vaporized organic liquid spacer in lines 80, 123 and 124 is .passed intoa header 126 which is connected to the top of collector 108. The composition of the organic liquid `spacer is maintained relatively constant by supplying thereto isobutane and propane at a constant rate. This is vaccomplished by passing a portion of the condensed jhydrocarbon from the auto-refrigerant system of the alkylation reactor to the collector 108. The condensedhydrocarbon contains propane and isobutane and this material is I.passed from the bottom of surge drum Li4-by means of vline4l6 such that a `portion is subjected to separation treatment in .the de propanizer mentioned -hereinabove and the remaining portion, namely, containing 80-90% .by volume of butane, is passed .through line 12S and .thereby enters the top .of collector 108. -ln .order to supply heat for melter 111 wherein acid crystals are melted to the .liquid form, the propane vapor stream of the depropanizer is passed through line 54 and enters heat exchanger 116 for indirect exchange of heat therein. The vaporous propane stream having a temperature of F. is reduced in temperature to 105 Fand thereby liquefied. The `cooled or liquefied propane stream passes from heat exchanger 116 and enters .line r130, and thence this material `is `.passed :to 'collector 108 by means .of line 131. Liqueiied 5propane which ,is produced in the depropanizer .column and is :passed .through line 62 enters the top of collector 108 by means of line 131. Any vaporized vhydrocarbon present within collector 108 is discharged overhead .therefrom by means of line 13S and this material .is passed vto .the auto-.refrigeration system of. the .alkylation reactor. In this respect, .the vaporized stream inline combines with the .vaporized hydrocarbon in line 38 prior to being compressed by means of compressor 40. .Organic liquid Aspacer is withdrawn from collector 108 by means of line 140, `and this material yis transported vby means .of pump 141 such that a portion is .utilized .as spacer and auto-,refrigerant inthe crystallizer 77 and the remaining portion is passed through line 91 for .the purpose of washing the acid `crystals which are present asa cake on .the rotating drum (not shown) of the centrifugal filter 9.0.

Having thus ,provided a written description of my invention, along with a specific example thereof, it should be understood that no undue limitations or restrictions Vare to be imposed .by reason thereof, but that the scope of the .invention is defined by the appended claims.

I claim:

l. In a process 'in which spent sulfuric acid is crystallized in a crystallization zone to produce a product comprising mother liquor `and crystals of sulfuric acid and in Which said product is Withdrawn from said crystallization zone, the method for preventing the agglomeration of said crystals which comprises injecting a portion of the product thus Withdrawn into said crystallization zone at a linear velocity of at least 50 feet per second to prevent agglomeration of the crystals.

2. In a process in which spent sulfuric acid is crystallized in a crystallization zone to produce a product comprising mother liquor and crystals of sulfuric acid and in which said product is Withdrawn from said crystallization zone, the method for preventing the agglomeration of said crystals which comprises injecting a portion of the product thus Withdrawn into said crystallization zone at a linear velocity of about l0() to about 4U() feet per second to prevent agglomeration of the crystals.

3. ln a process in which spent sulfuric acid is crystallized in the presence of an organic liquid spacer in la crystallization zone to produce sulfuric acid crystals, mother liquor and organic liquid spacer, the method of operating to prevent agglomeration of crystals which comprises injecting a slurry of sulfuric acid crystals, mother liquor and liquid spacer into said crystallization zone at a linear 10 velocity of at least 50 feet per second to prevent substantial separation of mother liquor from spacer within said crystallization zone.

4. ln a process in which spent sulfuric acid is crystallized in the presence of an organic liquid spacer in a crystallization zone to produce sulfuric acid crystals, mother liquor and organic liquid spacer, the method for preventagglomeration of crystals which comprises, injecting a slurry of sulfuric acid crystals, mother liquor and liquid spacer into said crystallization zone at a linear velocity of at least 50 feet per second and srnultaneouslqI mechanically agitating the contents of said crystallization zone to prevent substantial separation of mother liquor from spacer within said crystallization zone.

References Cited in the le of this patent UNITED STATES PATENTS Gensecke Mar. 17, 1936 Skelly et al Aug. 30, 1955 OTHER REFERENCES UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patat- Nm e 2,903,339 september e, .1959

vSalmlel Re Stiles Itis hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, linel 69, for "lne" read uw line w; column '7, lines 38 and 39, for "motor", each occurrence, read me mother me Signed and sealed this lQth doy of April 1960@ (SEAL) A Attest:

KARL E. AXJINE Commissioner of Patents 

1. IN A PROCESS IN WHICH SPENT SULFURIC ACID IS CRYSTALLIZED IN A CRYSTLLIZATION ZONE TO PRODUCE A PRODUCT COMPRISING MOTHER LIQUOR AND CRYSTALS OF SULFURIC ACID AND IN WHICH SAID PRODUCT IS WITHDRAWN FROM SAID CRYSTALLIZATION ZONE, THE METHOD FOR PREVENTING THE AGGLOMERATION OF SAID CRYSTALS WHICH COMPRISES INJECTING A PORTION OF THE PRODUCT THUS WITHDRAWN INTO SAID CRYSTALLIZATION ZONE AT A LINEAR VELOCITY OF AT LEAST 50 FEET PER SECOND TO PREVENT AGGLOMERATION OF THE CRYSTALS. 